A gel is a transparent, coloured plastic material used in photography for altering or correcting the colour of light. Gels are inexpensive and durable and an absolute must in any photographer's kit.
In my kit I have several gels. Red, blue, three shades of colour temperature orange (CTO) and green. The red and blue gels are for fun, the CTOs are for changing flash to match tungsten or adding warmth to skin tones and the green gel corrects for fluorescent lighting. You can typically find gels at your local photography store but any place that sells or rents movie studio or lighting equipment is likely to have some too. In Calgary I get them at a place called MTM Equipment Rentals where a one foot by two foot gel will run you $7 - cheap! That gives you an absolute ton of material for multiple flash heads.
Once you have the gel material you can cut it down to the size appropriate for your flash head. And to make attaching/detaching much easier and quicker, pick up some velcro tape. I prefer not to gum up my flash heads with velcro tape so instead I use the Honl speed strap. The speed strap provides the loop side of the velcro so I just apply the hook side of the velcro tape to the flash head and it is ready to go. I made up at least two sets of each colour.
If you are not a do-it-yourself kind of person you can always purchase gel correction and colour kits from Honl directly here. Of course Honl is not the only game in town but their speed straps and gels do work together nicely. I also have a Honl snoot and grid in my kit and although they are a bit pricey, they do work very well.
To give you an idea just how fantastic these gels are take a look at the photos below. The lighting is basically the same except in the first shot I turned off the background strobe. The background material was white seamless paper.
On the second shot I used a "079 Just Blue" gel manufactured by Lee Filters. Big difference hey? Lee's website has a ton of product information and tools to help you find precisely what you need. And they have this nifty "swatch ball" where you can check out their vast selection of coloured gels.
One of the great things about coloured gels is that they can turn your plain white background in to any colour you want. It saves you having to buy a different colour for every occasion.
Not only are gels invaluable for colour correction they are a cheap and easy way to add a little fun to your photography.